For most of the history of telecommunications, these physical links have comprised wire pairs (usually of copper, and often referred to as the “copper” network) over which electrical signals (which have included both analogue and digital systems at different stages of development) are carried.
The wires themselves are usually either suspended from poles or routed through underground conduits. Both types require some protection from their environment—in particular, overhead wiring requires reinforcement to support its own weight and the forces imposed on it by the wind, or any birds, ice, or debris that land on it. Underground wiring is protected from these forces, but requires protection against damage from ground movements and from excavation by animals or humans. Both types also require waterproofing, and electrical insulation between the two wires in each pair and from any other wire pairs using the same routing.
With the development of fiber optic communication systems in the distribution network, more complex arrangements are becoming necessary. In particular, it is more difficult to connect individual lengths of optical fiber. Instead, hollow tubes are provided, linked together as necessary to provide the complete connection from exchange to end user (or intermediate fiber/wired interface) through which an optical fiber may be introduced to make the actual connection, for example using the process described in European Patent 0108590.
Where a fiber network is being installed, it is often also necessary to make provision for “legacy” copper systems, either for “lifeline” backup services or because it is not possible or desirable to convert all customer premises from copper to fiber at the same time. It is therefore common for copper and fiber connections to be installed in parallel over much of the distribution network. The provision of both types of connection adds to the number of cables required to be provided in the underground ducting or on high-level connections between poles, requiring additional infrastructure and installation costs.
It is often necessary to provide electrical power to operate equipment at one or other end of the connection or at intermediate distribution points, such as an optical/copper interface. Traditionally, power was provided from the exchange, over the copper network. More recently, greater power requirements within the network have made it inefficient to use the traditional copper pairs for power supply from the exchange, so power injection over the copper network from the customer end, or from intermediate points, is becoming more common. The increasing use of optical fiber connections, which require electrical power to generate light signals but cannot be used to deliver electrical power, also increases the requirement for electrical connection to intermediate points in the network.
However, this increases the number of power connections required, increases exposure to tampering with the power supply at intermediate points in the network, and can be unpopular with customers. Moreover, if reliance is placed on collecting power from the customer to power the communications system, the customer's communications connection will fail if the power fails, making it difficult to report that failure to the power supplier.
It is therefore often desirable to provide an electrical connection, primarily configured to deliver electrical power, between the exchange and some point in the distribution network. By configuring it specifically for power rather than communications, some of the inefficiencies described above caused by carrying power over the traditional wire pairs can be avoided. In particular it is possible to use higher voltages, thereby reducing losses in the cable. However, this again requires extra installation work, and extra space in underground ducts or overhead cable installations.
Although topologically each connection (optical, power, or traditional “copper pair”) is a single run from the exchange to the customer premises, in practice it may be made up of two or more lengths in series, connected together at distribution points. Individual wire pairs or fiber tubes may be bundled into the same cable over some of these lengths, for example from the exchange to a first distribution point, at which point some or all of the individual connections may continue over different routings.
As distribution points are being placed deeper into the access network, running a separate, dedicated, cable all the way from the exchange to each distribution point is becoming impractical. In many cases such a cable would pass close to other distribution points, and it is therefore preferred to use a single multicore cable for the path to the first distribution point, at which point a second cable, using fewer cores, is connected to the first to provide a connection to the next distribution point. Several distribution points may be “daisy-chained” in this way. However, this requires the multicore cable to be cut and re-spliced at each distribution point, leading to multiple potential failure points in the cable.